1. Field of the Invention
The present invention generally relates to the measurement of network parameters with a multiport network analyzer and, more particularly, to a method for calibrating the analyzer for two port high frequency measurements.
2. Description of the Related Art
In designing cables and connecting hardware for use with high frequency signals it is imperative to understand the electrical characteristics of the components used to link a device or cable under-test with the electronics used to transmit and receive data. All of the possible couplings between connectors and cables can be measured with multiport measurement equipment comprising a network analyzer and a switching network. The key to providing an accurate multiport measurement system is a two port calibration procedure which is used repeatedly on every combination of pairs of ports in the measurement system.
Errors associated with network analyzer measurements can result from either non-ideal conditions in the measurement equipment or from the effects of the mechanical fixtures, such as cables and connectors, which are used to connect a device under test (DUT) to the network analyzer or test bed. A simple way to model this error is to view the test bed and the DUT as an ideal network analyzer cascaded with an error network and followed by the DUT. Such a network representation is illustrated in FIG. 1. The intervening network can then be used to account for the effects introduced by internal switching matrices connecting the various analyzer ports to the mechanical fixtures, as well as other sources of loss and distortion within the analyzer. The network can also be used to account for the effects of the fixtures themselves, which effects may introduce loss, crosstalk, and impedance discontinuities. Thus, the ideal network analyzer measures the cascaded network consisting of the intervening linear network followed by the DUT and obtains the measured S-parameters of the composite intervening network and DUT, here denoted as SM. (See FIG. 1).
During testing, the acquired data which is intended to characterize the DUT is oftentimes corrupted by imperfections in the measuring device. The data must therefore be processed to compensate for known errors when extracting the parameters which describe the DUT, here denoted as SA. An algorithm is used to relate the measured parameters SM to the actual parameters SA. However, because the intervening network in the model is partially internal to the measurement equipment, SA cannot be evaluated separately. The only way to characterize SA, therefore, is to measure various DUTs with known electrical characteristics and extract the appropriate parameters from SM. This process is further complicated when switching networks are involved because each change in the switch settings of the network analyzer results in a change in the intervening network due to differences in the signal routing. Thus, a different intervening network exists for each switch setting of the Network Analyzer, and each network must therefore be separately evaluated to compensate for measurement errors.
Existing calibration procedures involve very specific error models which do not account for the electrical properties of the jumpers used when the two measurement ports of the network analyzer are bridged as part of the calibration process. Although acceptable for low frequencies measurements, at high frequencies where the electrical length of the fixtures may easily approach a quarter wavelength, this failure introduces substantial errors which will not be corrected by the remaining calibration.
It would therefore be advantageous to provide a method for compensating for the inaccuracies introduced during the performance of two port calibration procedures which is used repeatedly on every combination of pairs of ports in a measurement system.
The above and other problems are overcome by a method for calibrating two port high frequency measurements which takes into account the electrical properties of jumpers which are used when the measurement ports of a network analyzer are bridged as part of the calibration process.
According to the invention, a method is used to remove systematic errors from two port network analyzer measurements. The method allows an intervening network to be completely arbitrary, and computes and factors the electrical properties of the jumpers which are used to bridge the analyzer ports of the network analyzer into a calibration process. This results in more accurate high frequency data than is possible with traditional calibration methods. This level of precision is essential when significant signal processing is applied to the measured data via software residing on a PC.
The calibration procedure consists of several stages and requires various calibration standards, typically an open circuit termination, a short circuit termination, and a reference load which is usually 50xcexa9 or 75xcexa9, for example. The calibration standards are first connected to port one and are individually measured. This process is repeated with a port two of the network analyzer. Next, a jumper (for later use in bridging ports one and two of the network analyzer) is connected to port one. At the far end of the jumper, the three reference calibration standards are sequentially connected and the combination of the jumper and a termination is measured. The raw data obtained thus far is then processed to extract the two port electrical parameters of the jumper. The measured jumper is then bridged between ports one and two of the network analyzer and a series of measurements are made. Using the known electrical characteristics of the jumper, the intervening network is completely characterized and the measured SM and actual parameters SA of any DUT are related.